The casting of integral gas turbine wheels having an equiaxed grain hub and directionally solidified columnar grain airfoils is described in U.S. Pat. No. 4 813 470. This patent describes a casting furnace having an annular chill that cooperates with a ceramic investment mold to form the columnar grain airfoils. Vibrators are provided proximate the central hub-forming region of the melt-filled investment mold to vibrate the mold in a manner that forms the equiaxed grain structure at the hub region of the cast turbine wheel.
Past practice in the casting of gas turbine wheels has involved preheating the ceramic investment mold in a mold heating furnace. The preheated mold then is moved by a mold handling mechanism (either manually or by assisted method), in ambient air, to a casting furnace. The furnace has a crucible that provides molten metal for casting under vacuum into the preheated mold and a chill that cooperates with the mold, thus forming columnar grain airfoils that solidify first in the mold followed by the equiaxed grain hub. This practice is disadvantageous in that considerable heat is lost from the preheated mold during transport from the the mold heating furnace to the casting furnace. This also makes mold handling difficult due to the high mold temperature typically used; and the necessity to accurately place the mold onto the chill.
An object of the present invention is to provide a mold heating vacuum casting furnace and method of casting that overcome these disadvantages.